2 What are? There are several different ways to define the terms acid base Each definition comes from a different acid/base theory: Arrhenius Theory Brønsted-Lowry Theory Lewis Theory Arrhenius Theory Acid: when dissolved in water, the hydrogen ion concentration increases (basically, an Arrhenius acid produces H + ions) Ex) HCl (aq) H + (aq) + Cl (aq) HCl is an acid b/c it is ionized (molecular acid that is pulled apart by water) to form H + Base: when dissolved in water, the hydroxide ion concentration increases (basically, an Arrhenius base produces OH ions) Ex) NaOH (aq) Na + (aq) + OH (aq) NaOH is a base b/c it dissociates (ionic compound breaking up into its ions) to form OH Problems with Arrhenius Theory Does not explain why molecular substances, like NH 3, some ionic compounds, like Na 2 CO 3 or Na 2 O, are bases since they do not contain OH ions, they can t be considered Arrhenius bases Does not explain why molecular substances, like CO 2, dissolve in water to form acidic solutions since it doesn t even contain H + ions it can t be considered an Arrhenius acid Does not explain acid-base reactions that take place outside of aqueous solutions 2

3 Hydronium Ion: H 3 O + the H + ions produced by the acid are so reactive they cannot exist in water H + ions are just protons w/ no valence electrons!!! To be more stable, H + ions react with water molecule(s) to produce hydronium ions, H 3 O + Note: for our purposes, H + H 3 O + will be used interchangeably Brønsted-Lowry Theory The Brønsted-Lowry theory looks at the entire acid-base chemical reaction In an acid-base reaction, an H + is transferred from the acid to the base Broader, more general, definition than Arrhenius does not have to take place in aqueous solution Brønsted-Lowry & Brønsted-Lowry acids are H + donors (aka: proton donors) Must have a removable (acidic) hydrogen because of the molecular structure, often one H in the molecule is easier to transfer than others (this is known as the ionizable hydrogen ) Brønsted-Lowry bases are H + acceptors (aka: proton acceptors) Must have a lone pair/ nonbonding pair of electrons because of the molecular structure, often one atom in the molecule is more willing to accept 9 H + transfer than others 3

6 Conjugate Pairs Practice Determine the acid, base, conjugate acid, conjugate base in the following reaction: H 2 O + H 2 SO 4 HSO 4 + H 3 O + base acid conjugate conjugate base acid Arrow Conventions chemists commonly use two kinds of arrows in reactions to indicate the degree of completion of the reactions a single arrow indicates all the reactant molecules are converted to product molecules at the end a double arrow indicates the reaction is in equillibrium. The reaction stops when some of the reactant molecules have been converted into products. Some reactant molecules remain some products molecules have formed Looks like in these notes Strong or Weak a strong acid is a strong electrolyte practically all the acid molecules ionize ( ) a strong base is a strong electrolyte practically all the base molecules form OH ions, either through dissociation or reaction with water ( ) a weak acid is a weak electrolyte only a small % of the molecules ionize ( ) a weak base is a weak electrolyte only a small % of the base molecules form OH ions ( ) 6

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